Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. A PD may be classified as class 0 to class 4. A PD of class 1 requires that the PSE supplies at least 4.0 W, a PD of class 2 requires that the PSE supplies at least 7.0 W, and a PD of class 0, 3 or 4 requires at least 15.4 W. Based on the determined class of the PD, the PSE applies the required power to the PD.
A semiconductor device, such as a MOSFET, may be used for controlling power from the PSE. During power-up and short-circuit conditions power dissipation in the MOSFET may be much higher than power dissipation when nominal power is provided. To limit the power dissipation, a foldback mechanism is prescribed by the IEEE 802.3af standard. In particular, the standard defines that during startup for port voltages between 10V and 30V, the minimum requirement for an output current in startup mode (IInrush) is 60 mA. For port voltages above 30 V, the current IInrush is required to be in the range from 400 mA to 450 mA. This 400 mA to 450 mA IInrush requirement applies for duration of the 50 ms to 75 ms TLIM timer.
FIG. 1 shows a diagram that graphically illustrates the IEEE 802.3af foldback requirements. In particular, the gray areas in FIG. 1 show combinations of PSE output voltages and output currents that are not allowed by the IEEE 802.3af standard. The black line in FIG. 1 illustrates a possible foldback curve representing the output current of the PSE at a level between 400 mA and 450 mA for output voltages above 30V and gradually reduced for output voltages below 30V.
However, for a high-power PSE capable of providing higher power (as opposed to higher voltages) than the 400 mA to 450 mA current prescribed by the IEEE 802.3af standard. Because higher currents cause higher MOSFET power dissipation, it would be desirable to have foldback that provides further reduction in power than afforded by the regular IEEE 802.3af foldback mechanism.
Therefore, there is a need for a programmable foldback mechanism capable of adjusting current limits to enable a PoE system to operate in a high-power mode and in a IEEE 802.3af compliant mode.